Re: [PATCH v6 1/3] memcontrol: define root_mem_cgroup for CONFIG_MEMCG=n cases

[Vlastimil Babka <vbabka@suse.cz>]

On 7/19/24 11:16 AM, Qu Wenruo wrote:
> There is an incoming btrfs patchset, which will use @root_mem_cgroup as
> the active cgroup to attach metadata folios to its internal btree
> inode, so that btrfs can skip the possibly costly charge for the
> internal inode which is only accessible by btrfs itself.
> 
> However @root_mem_cgroup is not always defined (not defined for
> CONFIG_MEMCG=n case), thus all such callers need to do the extra
> handling for different CONFIG_MEMCG settings.
> 
> So here we add a special macro definition of root_mem_cgroup, making it
> to always be NULL.
> 
> The advantage of this, other than pulling the pointer definition out,
> is that we will avoid wasting global data section space for such
> pointer.
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---
>  include/linux/memcontrol.h | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 030d34e9d117..a268585babdc 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -329,8 +329,6 @@ struct mem_cgroup {
>   */
>  #define MEMCG_CHARGE_BATCH 64U
>  
> -extern struct mem_cgroup *root_mem_cgroup;
> -

This breaks the CONFIG_MEMCG=y case?

>  enum page_memcg_data_flags {
>  	/* page->memcg_data is a pointer to an slabobj_ext vector */
>  	MEMCG_DATA_OBJEXTS = (1UL << 0),
> @@ -346,6 +344,12 @@ enum page_memcg_data_flags {
>  
>  #define __FIRST_OBJEXT_FLAG	(1UL << 0)
>  
> +/*
> + * For CONFIG_MEMCG=n case, still define a root_mem_cgroup, but that will
> + * always be NULL and not taking any global data section space.
> + */
> +#define root_mem_cgroup		(NULL)
> +
>  #endif /* CONFIG_MEMCG */
>  
>  enum objext_flags {